Add entry about stateful buttons to v4 migration guide

Refs #18459, #17178
[skip sauce]

docs/migration.md

@@ -85,6 +85,8 @@ New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with
 
 - Renamed `.btn-default` to `.btn-secondary`.
 - Dropped the `.btn-xs` class entirely.
+- The [stateful button](http://getbootstrap.com/javascript/#buttons-methods) feature of the `button.js` jQuery plugin has been dropped. This includes the `$().button(string)` and `$().button('reset')` methods. We advise using a tiny bit of custom JavaScript instead, which will have the benefit of behaving exactly the way you want it to.
+  - Note that the other features of the plugin (button checkboxes, button radios, single-toggle buttons) have been retained in v4.
 
 ### Grid system